1. Field of the Invention
The present invention relates to a belt-and-pulley type continuously variable speed, and more particularly to a belt-and-pulley type continuously variable transmission having a drive pulley on an input shaft, a driven pulley on an output shaft, each of the drive and driven pulleys comprising stationary and movable conical members, and a V-belt trained around the drive and driven pulleys, the movable conical members of the drive and driven pulleys being operated by a servo hydraulic actuator for controlling the effective diameters of the pulleys thereby to continuously vary the transmission ratio between the input and output shafts, i.e., the drive and driven pulleys.
2. Description of the Relevant Art
The aforementioned transmissions for use on motor vehicles are required to transmit the torque effectively by tensioning a V-belt trained around drive and driven pulleys to a degree proportional to the torque to be transmitted as by applying side force to the V-belt. The V-belt should remain tensioned while the effective diameters of the drive and driven pulleys are to be varied in order to vary the speed ratio.
Japanese Laid-Open Patent Publications Nos. 55-65755 (published May 17, 1980) and 60-125450 published July 4, 1985) disclose the aforementioned types of V-belt-and-pulley-type continuously variable transmissions in which the oil pressure from a pump is regulated according to the rotational speed of a driver pulley (i.e., the rotational speed of an engine) and a speed ratio, and the regulated oil pressure is supplied through a slide valve to a servo hydraulic pressure chamber for axially moving a movable conical member of the drive pulley so that the effective diameter of the driver pulley can be controlled while keeping a side force imposed on the V-belt. The slide valve is controlled by the rotational speed of the drive pulley and the opening of a throttle valve.
In the aforementioned transmissions, however, the effective area in the servo hydraulic pressure chamber for actuating the movable conical member of a driven pulley is smaller than that in the servo hydraulic pressure chamber for actuating the movable conical member of the drive pulley, and the hydraulic pressure chamber associated with the driven pulley is always supplied with regulated oil pressure. In order to slide the movable conical member of the drive pulley in the direction of a smaller speed ratio, the effective diameter of the drive pulley is increased so that the tension of the V-belt is increased to move the movable conical member of the driven pulley against the side force thereon for thereby reducing the effective diameter of the driven pulley. Therefore, the load on the V-belt is increased, affecting the durability of the V-belt. Conversely, in order to slide the movable conical member of the drive pulley in the direction of a larger speed ratio, the oil pressure in its servo hydraulic pressure chamber is released to decrease the tension of the V-belt so that the side force on the driven pulley is utilized to move its movable conical member against the tension of the V-belt for thereby increasing the effective diameter of the driven pulley. In this mode of operation, the side force on the drive pulley may unnecessarily be reduced.
U.S. Pat. No. 3,600,961 issued Aug. 24, 1971 discloses a control structure utilizing higher-pressure oil of a constant pressure and regulated lower-pressure oil which can be regulated by a speed ratio or the like, the lower-force being applied to impose a side pressure on drive and driven pulleys. When the speed ratio is to be changed, the higher-pressure oil is selectively supplied to a servo hydraulic pressure chamber associated with one pulley the effective diameter of which is to become larger. The disclosed arrangement is also disadvantageous in that the load on a V-belt is increased to the extent which affects the durability thereof. In some speed ratio range, a higher side force than the necessary side force has to be applied, resulting in that an unnecessarily higher oil pressure is required.
The two Japanese Laid-Open Patent Publications, referred to above, may be also disadvantageous in that when the servo hydraulic pressure chamber of the drive pulley is connected to a drain passage to lower the pressure in the changer to thereby increase the speed ratio, most of the oil is drained from the servo hydraulic pressure chamber and air is allowed to enter the servo hydraulic pressure chamber, whereupon the drive pulley fails to apply the desired side force on the belt. Moreover, when regulated oil pressure is again introduced into the servo hydraulic pressure chamber, air remaining therein is compressed before the speed ratio starts to be changed. As a result, the transmission may not operate with a good response, when the speed ratio thereof is to be again decreased.
Japanese Laid-Open Patent Publication No. 60-159456 published Aug. 20, 1985 discloses the aforementioned type of belt-and-pulley type continuously variable transmission having drive and driven pulleys associated with servo hydraulic pressure chambers which have effective areas, respectively, different from each other, the hydraulic pressure chamber of the driven pulley being supplied with regulated oil pressure at all times. When the hydraulic pressure in the hydraulic pressure chamber of the drive pulley is low, low-pressure oil is supplied thereinto to prevent oil from being drained. Therefore, the disclosed transmission is free from one of the drawbacks as described just above. However, the transmission shown in this publication suffers from the following shortcomings:
1. Where the transmission is employed in a motor vehicle, mutually related signal oil pressures representing the opening of the throttle valve and the rotational speed of the engine are applied onto the opposite ends of a slide valve, and the stationary port of a servo hydraulic pressure chamber is connected selectively to an oil pressure passage and an oil drain passage for varying the speed ratio. In such an arrangement, however, there are a speed range in which an upshift and/or downshift cannot be performed resulting in that the speed ratio cannot freely be controlled in some speed ranges. PA1 2. The speed at which the speed ratio varies tends to be constant at all times.
The above problems 1 and 2 can be solved by a belt-and-pulley type continuously variable transmission disclosed in Japanese Laid-Open Patent Publication No. 60-136655 published July 20, 1985. In this disclosed transmission, a regulated oil pressure is supplied to the hydraulic pressure chamber of a drive pulley through a directional control valve and a speed control valve which are disposed hydraulically in series with each other and controlled by solenoid-operated valves, respectively. The directional control valve disposed upstream of the speed control valve serves to change the direction in which the speed ratio varies, and the downstream speed control valve serves to control a speed at which the speed ratio varies. According to the disclosed transmission, the effective area of the movable conical member of a driven pulley is smaller than that of the movable conical member of the drive pulley, and the hydraulic pressure chamber of the driven pulley is supplied with the regulated oil pressure at all times. Therefore, the first mentioned problem that the load acting on the V-belt increases when varying the speed ratio cannot be solved by this transmission.
The present invention has been made in order to effectively solve the problems of the aforementioned transmissions for use in motor vehicles.